The invention is in the field of the packaging industry and relates to a method according to the generic part of the independent claim and to packaging materials produced according to the inventive method. The method serves for manufacturing packaging materials consisting of a polyolefin foam layer which is coated with a coating film on at least one side.
Packaging materials with a foam layer of e.g. polypropylene, in the form of quasi endless, rolled webs are used for producing thermoformed, self supporting packaging items as e.g. trays for food, which trays are, after filling e.g. closed with a transparent film. Such materials are applicable in the so called xe2x80x9cForm-Fill-Sealxe2x80x9d packaging method (FFS-method). For this method the packaging material is fed into an apparatus in which it is formed into a web of trays, which trays in the same apparatus are filled, sealed with a sealing film and then are separated from each other.
In these packaging materials the polypropylene foam layer is coated either on one side or on both sides with a coating film. The foam layer renders the packaging item stiff and the coating film basically closes the pores of the foam layer in order to make it tight and to prevent liquid to enter into the inside of the foam material. Depending on the composition and thickness of the coating film, this film may also serve as a further means for increasing the stiffness, as protection of the foam surface and/or as gas and aroma barrier. Usually the coating film carries on its surface facing away from the foam layer a sealing layer which serves as a bonding layer between the packaging material and a transparent film used for closing the packaging item (tray).
Packaging materials with a polypropylene foam layer for producing trays by thermoforming are described e.g. in the publication EP-A1-0570222. The described materials are manufactured by thermobonding (lamination by application of heat and pressure) to a sheet of a polypropylene foam, a multilayer film, which multilayer film includes a barrier layer rendering the packaging material gas- and/or aroma-tight. This multilayer film needs a bonding layer facing the foam sheet which bonding layer has to consist of a material capable of adhesion to the foam sheet in the thermobonding process, i.e. when exposed to heat and pressure. The heat which may be used for the bonding is limited by the polypropylene foam which is to be laminated without change to its structure by the heat applied. This means that the bonding layer of the coating film needs to be made of a polymer plasticating at a considerably lower temperature than polypropylene, e.g. an ethylene copolymer.
The low melting bonding layer renders the whole structure rather thermo-sensitive and represents a foreign material regarding the recyclability of the polypropylene material. Furthermore it renders the choice of materials for the foam layer rather narrow because there has to exist a material for the bonding layer plasticating at a lower temperature than the foam material and still satisfying the minimal demand on thermal stability which is posed on the packaging items made from the packaging material. Furthermore the demands on the material of the bonding layer render the coating film structure complicated as can be seen from the example given in the above mentioned publication.
In the publication EP-344726 similar packaging materials are described, which packaging materials consist substantially of a foamed plastic (e.g. polypropylene) coated on one or both sides. For producing the material adhesion methods (as EP-0570222) are proposed or coextrusion methods. For the coextrusion method, the foamed layer is coextruded with additional layers either of the same plastic material as the foamed material or of a different plastiac material.
It is the object of the invention to create a method for manufacturing packaging materials with a polyolefin foam layer applicable for forming self supporting packaging items such as trays for food packaging, which method allows to omit a bonding layer with a substantially lower thermostability than the foam layer has. Using the inventive method allows therefore a larger choice of polyolefins for the foam layer than do known such methods. The packaging material produced by the method is to contain less material different from the foam material than known such packaging materials do. The method is not to be connected with more expenditure than known methods for manufacturing packaging materials with a polyolefin foam layer usable for the same purposes.
The above objects are achieved by the method for producing packaging materials with a polyolefin foam layer as defined by the patent claims.
The inventive method is based on an extrusion lamination step for coating the polyolefin foam layer.
Thereby, a foam sheet produced in a separate method step by extrusion and expansion is coated by extrusion lamination with a single-layer or a multilayer coating film produced in a separate method step by coextrusion. This means that the polyolefin foam sheet and the coating film are guided together and a further bonding layer is extruded between them. This further bonding layer extruded in the extrusion lamination step consists of a polyolefin based on the main monomer of the foam layer. Immediately after the extrusion of the further bonding layer, there is sufficient pressure applied to the composition to achieve a satisfactory bond without impairing the foam layer. The coating film needs a bonding layer facing towards the foam layer (or rather toward the further bonding layer), which consists of a polyolefin based on the monomer which is pressuring the main monomer of the polyolefin of the foam layer (e.g. propylene) also. This bonding layer again may be the only layer of a single layer coating film or it may be the innermost layer of a multilayer coating film.
The packaging material produced according to the inventive method always shows on the surface of the foam layer two layers of basically the same polyolefin as the polyolefin of the foam layer.
If the foam layer is to be coated on both sides, this can be done by producing the foam sheet and the two coating films in separate method steps and by coating the foam layer in two extrusion lamination steps. It can further be done by producing a foam layer coated on one side by coextrusion and coat this layer on the other side by extrusion lamination with a coating film produced in a separate coextrusion or extrusion step. In all cases, on the coated surface of the polyolefin foam layer there are two layers of basically the same polyolefin as the polyolefin of the foam layer.